MDCT Technology Kalpana M. Kanal, Ph.D., DABR Assistant Professor Department of Radiology University of Washington Seattle, Washington ACMP Annual Meeting 2008 - Seattle, WA Educational Objectives Historical Development of CT Why has the frequency of CT increased? Multi-Row Detector CT and Pitch Advantages of Multi-Row Detector CT Automatic Exposure Control in CT Cardiac CT Future of CT Kanal 2 Historical Development of CT 64 slice scanners Dual Source CT 2004- 2005 Slow scan time Fast scan time Dr. Mahesh, John Hopkins, MD, AAPM Handout. Kanal 3 Historical Development of CT Seventh Generation CT (late 1990’s – 2000) Multi-Row Detector Systems Radiographics 2000: 2020-17871787-1806 Kalendar WA, Computed Tomography, Second Edition, 2005, pg 58. Kanal 4 Historical Development of CT PET/CT Scanners (2001) Kanal Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 74. 5 Historical Development of CT Dual-Source CT (2005) A scanner using two X-ray sources and two detectors at the same time Enables scanning a heart at any heart rate and at the lowest radiation dose possible Kanal www.medical.siemens.com 6 Fast Scanning, 0.3 sec Kanal 7 Why has the frequency of CT exams increased? New technology Faster scans New exams done with CT More screening exams UNSCEAR 2000 More like 60% Kanal ICRP Publication 87 (2001) 8 Single-Row Detector versus Multi-Row Detector CT Kanal Radiographics 2000: 2020-17871787-1806 9 Single-row Detector vs. Multi-row Detector CT Kanal Seeram, Computed Tomography, 2nd ed., 2001, pg 258. 10 Multiple Detector Arrays Kanal Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 341. 11 Slice Width Selection GE 64 MDCT (40 mm detector) 64 x 0.625 mm 32 x 1.25 mm 16 x 2.5 mm 8 x 5 mm http://www.impactscan.org/slides/impactcourse/helical_and_multi-slice_principles/img17.html Kanal 12 32/40/64 Slice CT Systems http://www.impactscan.org/slides/ukrc2005/img11.html Kanal 13 Pitch in Multi-slice CT Two pitch definitions seen in MSCT Collimator pitch = table travel_____ x-ray beam width Detector pitch = table travel detector width Collimator pitch = Detector pitch N Kanal c.f. http://www.impactscan.org/slides 14 Multi-Row Detector CT - Advantages Same acquisition in shorter times (fewer motion artifacts) Kanal Seeram, Computed Tomography, 2nd ed., 2001, pg 263. 15 Multi-Row Detector CT - Advantages Reconstruction in different slice widths Kanal http://www.impactscan.org/ 16 Multi-Row Detector CT - Advantages Efficient use of x-ray beam with increasing slices Reduction of radiation dose Kanal Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 168. 17 Multi-Row Detector CT - Advantages Thinner slices for better z-axis resolution Isotropic Imaging Improved Spatial Resolution All sides of the voxel have equal dimensions Kanal Seeram, Computed Tomography, 2nd ed., 2001, pg 264. 18 Multi-Row Detector CT - Advantages Better Multiplanar Reconstruction & 3D Display Kanal Radiographics 2002: 2222-12551255-1268 19 Regarding Multiple Detector CT scanners: 14% 1) A scanner with a 16-detector array employs a total of 16x16 detectors 14% 2) Detectors used are the non-radioactive xenon-gas detectors 3) The slice width is determined by width of detectors in slice thickness dimension The slice width selected can be smaller than the physical size of the detector The spatial resolution is worse than that of a single-detector CT scanners 29% 43% 4) 0% Kanal 5) 20 Answer The slice width is determined by width of detectors in slice thickness dimension Kanal 21 Multi-Row Detector CT - Advantages Cardiac CT 40 mm detector (Pitch ~0.25) 12.5 cm in 5 sec 20 mm detector (Pitch ~0.25) 6.2 cm in 5 sec Kanal Kalendar WA, Computed Tomography, 2nd ed., 2005, Video. 22 For a 16-detector row CT scanner, the x-ray beam width is 20 mm, detector width is 5 mm and the table travel per rotation is 20 mm. Calculate the detector pitch and collimator pitch? 15% 62% 15% 8% 0% Kanal 1. 2. 3. 4. 5. 1, 4 4, 1 5, 20 4, 4 20, 5 23 Answer 4, 1 Pitchd = 20 = 4 = Detector pitch 5 Pitchx = 20 = 1 = Collimator pitch 20 Kanal 24 Dose Reduction Current Modulation or AEC in CT Modern CT scanners are capable of modulating the mA (current) during the scan Rationale: it takes fewer photons (lower mA) to penetrate thinner tissue, and more x-ray photons (higher mA) to penetrate thicker projections through the body Benefits Consistent image quality Dose reduction potential Reduced tube loading Extended scan runs Reduction in photon starvation artifact Kanal 25 Automatic Exposure Control (AEC) in CT Patient size AEC Patient size AEC: higher mA used for larger patients Kanal MHRA Report 05016, February 2005 www.isteve.com www.yuzbasicafer1.com 26 Automatic Exposure Control (AEC) Z-axis AEC Z-axis AEC: higher mA used at more attenuating positions along the z-axis (patient) Kanal MHRA Report 05016, February 2005 Radiographics, 2006, 26:503-512 27 Automatic Exposure Control (AEC) in CT Angular AEC Angular AEC: the degree of modulation depends on the asymmetry of the patient Kanal MHRA Report 05016, February 2005 Radiology, 2004, 233:649-657 28 Dose Reduction Cardiac ECG dose modulation It is possible to vary the mA based on R-R interval phase % This can lead to a dose decrease of up to 50% in low stable heart rates 70% mA 80% interval used for image reconstruction dose not given to patient 550 200 t Kanal 29 Dose Reduction - ECG dose modulation 90 HR 30-74 BPM (One Segment) 80 Exposure CTDI (mGy) 70 60 50 40 30 20 Segment mode, no ECG Coverage during scan 10 0 0 50 100 150 200 250 240o Heart rate (BPM) Kanal Paul Kinahan and Kelly Branch, UWMC 30 Dose Reduction - ECG dose modulation HR 30-74 BPM (One Segment), TR 175 msec 90 Exp o su re C T D I (m G y) 80 70 42% decrease in dose 60 50 40 30 Needed coverage during scan Segment mode, no ECG 20 Segment mode [65-85%], w/ECG 10 Segment mode [70-80%], w/ECG 0 0 Kanal 50 100 150 Heart rate (BPM) 200 250 240o Paul Kinahan and Kelly Branch, UWMC 31 Regarding automatic tube current modulation in CT: 0% 1. It gives inconsistent image quality 0% 2. It increases the scan time for the exam 0% 3. 4. It generates more heat in the x-ray tube The tube current is automatically adjusted according to patient size It causes an increase in photon starvation artifact 100% 0% Kanal 5. 32 Answer The tube current is automatically adjusted according to patient size Kanal 33 Reducing Cardiac Dose – Prospective Gating No functional information Kanal Mahesh, et al. Radiographics, 2007; 27:149527:1495-1509. Retrospective Gating Kanal Mahesh, et al. Radiographics, 2007; 27:149527:1495-1509. Prospective vs. Retrospective Gating University of Washington Study Effective Dose (mSv) Retrospective Gating 26.4 ±6.2 Prospective Gating 6.2 ± 2.0 77% lower radiation dose with prospective study This study concluded that the “64 channel cardiac CT performed with prospective ECG gating may have similar subjective image quality and substantially lower patient radiation dose compared to retrospective ECG gating” Kanal Shuman, et al. submitted for publication, 2007 Regarding prospective ECG triggering for cardiac CT exams 0% 1. Projection data has to be collected through out the cardiac cycle 0% 2. The temporal resolution is about 350 ms 3. The radiation dose is reduced compared to retrospective 100% ECG gating 0% 4. 0% 5. Kanal Functional information of the heart is available with this exam Increase in motion artifacts occurs 37 Answer The radiation dose is reduced compared to retrospective ECG gating Kanal 38 Future of CT Kanal Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 235. 39 Future of CT Other Developments Kanal Kalendar WA, Computed Tomography, 2nd ed., 2005, pg 72. 40 TAKE HOME POINTS MDCT scanners are becoming common and have several advantages over SDCT CT utilization has increased significantly AEC or Automatic Tube Current Modulation and ECG modulation available on modern scanners which helps reduce dose Cardiac CT is becoming popular with MDCT scanners Always keep ALARA in mind! Kanal 41 THANK YOU Kanal 42